1. Field of the Invention
The invention relates to methods of using polylactate release compounds.
2. Description of the Related Art
Compounds which release hydroxy acids slowly over time preferably xcex1-hydroxy acids can serve as a time-release source of lactic acid for biodegradation of chemical compounds in various media, including soils, aquifers, bioreactors, wastestreams, industrial processes, and other systems. The compounds may also be the basis of formulations which provide a time-release source of lactic acid and other materials and compounds which stimulate growth of microbes and facilitate bioremediation. The lactic acid, which is itself a nutrient for microbes, is broken down to form other compounds which provide both additional nutrients and a source of electrons to support the microbial biodegradation of chemical compounds, preferably halogenated hydrocarbons.
Halogenated hydrocarbons are compounds composed of hydrogen and carbon with at least one hydrogen substituted by a halogen atom (e.g. Cl, Br, or F). Halogenated hydrocarbons are used for many purposes, such as solvents, pesticides, and degreasers. Degreasing products have widespread use in several industries, including dry cleaning, microelectronics, and equipment maintenance. Some of the most common halogenated hydrocarbons are methylene chloride, chloroform, carbon tetrachloride, tetrachloroethane (TCA), tetrachloroethene (PCE), trichloroethene (TCE), dichloroethene (DCE), and vinyl chloride (VC). Such compounds are commonly known as xe2x80x9cchlorinated hydrocarbonsxe2x80x9d or xe2x80x9cchlorinated solvents.xe2x80x9d
Chlorinated hydrocarbons have been widely used for several decades. This use, in addition to improper handling and storage, has led to extensive soil and groundwater contamination, and these solvents are among the most prevalent groundwater contaminants in the United States today. Contamination of groundwater by chlorinated hydrocarbons is an environmental concern because these compounds have known toxic and carcinogenic effects.
One common technique for decontaminating aquifers that is in current use is the pump-and-treat method. As practiced, this method utilizes a series of extraction wells drilled into a contaminated aquifer. Contaminated water is drawn through an extraction well, treated to remove or degrade the contaminant, and then returned to the aquifer through one or more injection wells or discharged to sewers or other points of non-origin. This method can be time consuming and cost-prohibitive.
Recently, attempts have been made to biodegrade chlorinated solvents in-situ using anaerobic bacteria. Some species of anaerobic bacteria used in bioremediation of chlorinated solvents degrade these solvents by reductive dechlorination. This reductive process requires a steady supply of an electron donor such as hydrogen. Some current research supports the proposition that delivery of hydrogen in a slow, steady manner is an effective way to stimulate and maintain organisms that perform reductive dechlorination and reduce competition for ambient hydrogen by other organisms. Several methods have been proposed to supply the hydrogen needed for reductive dechlorination: addition of short chain organic acids or alcohols; addition of sodium benzoate (as disclosed in U.S. Pat. No. 5,277,815); addition of fats and oils; sparging with hydrogen gas (as disclosed in U.S. Pat. No. 5,602,296); and generating hydrogen gas in-situ by electrochemical reactions or electrolysis (also disclosed in U.S. Pat. No. 5,602,296).
All of the previously mentioned methods have serious shortcomings. Addition of short chain organic acids or alcohols as well as the addition of simple organic esters or organic salts such as sodium benzoate have the problem that essentially all of the chemical is released at once in the area and is free to flow away from the contaminated area. Thus, frequent addition of the chosen compound is needed to keep a sufficient concentration of the compound in the contaminated area over time. The constant injection of high volumes of solutions will increase the volume of the system or aquifer and thereby potentially cause further spread of the contamination. Furthermore, unless special measures are taken to deoxygenate the water and solutions which are injected, the level of oxygen in the system or aquifer will rise, thus harming the anaerobic atmosphere which fosters the microbes performing the reduction.
Sparging with hydrogen requires the installation and use of pipes, manifolds, valves, and other equipment and the handling of large quantities of a highly flammable and explosive gas under pressure. Generation of hydrogen gas in-situ by chemical reaction or electrolysis as disclosed in U.S. Pat. No. 5,602,296 is, by those inventors"" own admission, experimental in nature and like sparging suffers from the additional limitation in that hydrogen gas has very low solubility in water. Lastly, addition of fats and oils can provide for the slow release of hydrogen, but the method does not provide a mechanism for controlling the amount of hydrogen released. Furthermore, the amount of hydrogen released is very low compared to the weight of fat or oil that must be added.
One of the most effective substrates to provide hydrogen to a biological system is lactic acid. During anaerobic processes the conversion of lactic acid (or lactate salt) to acetic acid (or acetate salt) liberates two moles of dihydrogen (four moles of elemental hydrogen) for each mole of lactic acid or lactate consumed. 
Thus the process produces both an electron source (hydrogen) and a nutrient source for bacteria.
A convenient method of delivering lactic acid is in the form of an ester. Esters of lactic acid hydrolyze to produce free lactic acid, or lactate salt, depending on the pH of the solution. 
The hydrolysis reaction can be catalyzed by either acid or base, and the alcohol produced can also serve as a nutrient source for surrounding bacteria. The rate of hydrolysis is dependent upon both the pH and the alcohol with which the ester was formed. Although simple esters of lactic acid, such as ethyl lactate, delay the release of free lactic acid into solution, the lactic acid is still released and converted to hydrogen at a very high rate. This rate may be higher than the rate at which bacteria performing reductive dechlorination can consume it, and thus either be wasted or used by other bacteria which compete with the reductive dechlorinators.
The preferred embodiments relate to compounds, characterized by their ability to release hydroxy acids slowly over time. The preferred embodiments also relate to formulations comprising the compounds, as well as methods for their use in aiding bioremediation of media contaminated by contaminants capable of being remediated by microbial reduction.
In one aspect, the preferred embodiments provide for a composition comprising a multifunctional alcohol ester of a poly(hydroxy acid), wherein the poly(xcex1-hydroxy acid) is either a xcex1-hydroxy acid or a xcex2-hydroxy acid, and each hydroxyl group on the multifunctional alcohol has reacted to form an ester bond with a molecule of poly(hydroxy acid).
In the preferred embodiments, the poly(hydroxy acid) is an xcex1-hydroxy acid. In especially preferred embodiments, the composition has the formula: 
wherein n=1 to 4, m=0 to 3, and x=1 to9.
The preferred embodiments also provide a formulation comprising 65-99% by weight of a multifunctional alcohol ester of poly(hydroxy acid) and 1-35% by weight inorganic salts. Another formulation comprises 14-98% by weight of a multifunctional alcohol ester of poly(hydroxy acid), 1-15% by weight inorganic salts, and 1-85% by weight of a diluent which does not interfere with the hydrolysis of an ester. Preferably, the diluent is selected from the group consisting of water, glycerin, esters, and alcohols. In other embodiments, the formulations above further comprise 0-30% by weight of one or more compounds selected from the group consisting of nutrients such as yeast extract, urea, potassium-containing compositions, nitrogen-containing compositions, phosphorous-containing compositions, sulfur-containing compositions, molybdenum salts, iron salts, zinc salts, copper salts, buffers and pH modifiers such as sodium carbonate and potassium carbonate, ethylene, chelating agents, surfactants, vitamins such as B12, enzymes such as lipase and esterase, compounds that inhibit competing microorganisms, and bacteria and other microbes
Especially preferred compounds include glycerol tripolylactate, xylitol pentapolylactate, and sorbitol hexapolylactate.
There is also provided a process of making multifunctional alcohol esters of poly(xcex1-hydroxy acids) comprising the steps of charging a reaction vessel with solution of xcex1-hydroxy acid; adding a catalytic amount of a strong inorganic acid; heating the reaction vessel to drive off water and cause polymerization resulting in poly(xcex1-hydroxy acid); adding a multifunctional alcohol to the reaction vessel; heating the reaction vessel to cause esterification of the poly(xcex1-hydroxy acid); and adding an inorganic base to neutralize at least some of the inorganic acid in the reaction vessel. In embodiments wherein the reaction vessel has a large volume, the heating step to drive off water is preferably done under vacuum. The above process may further comprise steps wherein a solvent is added with the xcex1-hydroxy acid and the solvent is removed following addition of the inorganic base.
The preferred embodiments also provide for a method of aiding bioremediation of contaminants remediated through microbial reduction in a medium, comprising contacting the medium with applying a composition comprising an ester of an xcex1-hydroxy acid. In preferred embodiments, the xcex1-hydroxy acid is polymerized to form a poly(xcex1-hydroxy acid). In other preferred embodiments, the composition comprises a multifunctional alcohol ester of poly(xcex1-hydroxy acid) wherein each hydroxyl group on the multifunctional alcohol has reacted to form an ester bond with a molecule of poly (xcex1-hydroxy acid). The method may also utilize formulations, as described above, which comprise the poly(xcex1-hydroxy acid)esters. The medium is preferably selected from the group consisting of an aquifer, a bioreactor, soil, an industrial process, a wastestream, a body of water, a river, and a well.
When the medium is underground, the preferred method of aiding bioremediation comprises injecting the composition or formulation into the medium with a high pressure pump. Another preferred method comprises the steps of packing the composition into tubes or canisters having holes or slits in the sides thereof, and placing the canisters into holes drilled into the ground.
There is provided a method of aiding remediation of chemical compositions in a medium, comprising applying a polylactate ester to the medium. Preferably the contaminants are selected from the group consisting of nitrogen-containing organic compounds, oxygen-containing organic compounds, polyaromatic hydrocarbons, and halogen-containing organic compounds. More preferably, the contaminants comprise chlorinated aromatic or aliphatic hydrocarbons. In preferred embodiments, the polylactate ester is glycerol tripolylactate, xylitol pentapolylactate, and sorbitol hexapolylactate. The medium is preferably selected from the group consisting of an aquifer, a bioreactor, soil, an industrial process, a wastestream, a body of water, a river and a well.